Moving range gate generating system for radar apparatus

ABSTRACT

1. In a radar system for searching and tracking objects in space, a voltage comparator circuit comprising: a source of serially generated time base pulses having a predetermined pulse repetition frequency, bias means coupled to said pulse source and controlled by said pulses for deriving a control voltage extended over a predetermined duration, a first sawtooth generator biased by said control voltage to produce for said predetermined duration a sawtooth voltage encompassing a period greater in length than a plurality of periods of said pulse repetition frequency, a second sawtooth generator, multivibrator means normally having a first quiescent state and one output level and being switchable to a second state and another output level by each of said pulses, said second sawtooth generator being controlled by said multivibrator means, an input connection between said source and said multivibrator means whereby in response to the switching of said multivibrator means from said first to said second state said second sawtooth generator produces a second sawtooth voltage, a comparator having first and second input electrodes and an output electrode and characteristics of producing an output pulse on said output electrode each time the signals simultaneously applied to said input electrodes agree in amplitude, cathode follower means for applying said first sawtooth voltage to the first input electrode of said comparator, circuit means for applying said second sawtooth voltage to the second input electrode of said comparator, means connected between said output electrode and said multivibrator means for restoring the initial quiescent state of said multivibrator means upon the occurrence of said output pulses, whereby said second sawtooth voltage undergoes termination upon each reversal of said multivibrator means from said second state to said first quiescent state, the improvement lying in gradually increasing the delay between each of said output pulses and the related one of said time base pulses which spatially precedes it.

United States Patent [1 1 Hassencahl MOVING RANGE GATE GENERATING SYSTEMFOR RADAR APPARATUS [75] Inventor: Lloyd J. Hassencahl, Chatsworth,

Calif.

[73] Assignee:, The United States of America as represented by theSecretary of the Air Force, Washington, DC.

[22] Filed: Jan. 13, 1965 21 ApplfNo; 425,357

Primary ExaminerT. l-l. Tubbesing Attorney-Harry A. Herbert, Jr. andEugene J. Pawlikowski EXEMPLARY CLAIM 1. In a radar system for searchingand tracking objects in space, a voltage comparator circuit comprising:a source of serially generated time base pulses having a predeterminedpulse repetition frequency, bias means coupled to said pulse source andcontrolled by said pulses for deriving a control voltage extended over apredetermined duration, a first sawtooth generator bi- Dec. 25, 1973ased by said control voltage to produce for said predetermined durationa sawtooth voltage encompassing a period greater in length than aplurality of periods of said pulse repetition frequency, a secondsawtooth generator, multivibrator means normally having a firstquiescent state and one output level and being switchable to a secondstate and another output level by each of said pulses, said secondsawtooth generator being controlled by said multivibrator means. an

7 input connection between said source and said multivibrator meanswhereby in response to the switching of said multivibrator means fromsaid first to said second state said second sawtooth generator producesa second sawtooth voltage, a comparator having first and second inputelectrodes and an output electrode and characteristics of producing anoutput pulse on said output electrode each time the signalssimultaneously applied to said input electrodes agree in amplitude,cathode follower means for applying said first sawtooth voltage to thefirst input electrode of said comparator, circuit means for applyingsaid second sawtooth voltage to the second input electrode of saidcomparator, means connected between said output electrode and saidmultivibrator means for restoring the initial quiescent state of saidmultivibrator means upon the occurrence of said output pulses, wherebysaid second sawtooth voltage undergoes termination upon each reversal ofsaid multivibrator means from said second state to said first quiescentstate, the improvement lying in gradually increasing the delay betweeneach of said output pulses and the related one of said time base pulseswhich spatially precedes it.

.missile acquisition of momentarily lost targets and, particularly, to asystem for generating a movable range gate from fixed zero rangereference pulses.

The increasing use of electronic countermeasures equipment in modernweapons systems has fostered the need for target reacquisition methodsto be used at timeswhen enemy jamming has temporarily broken thetracking lock which the guidance system of the missile has establishedon the target. Missile tracking and guidtance systems areespeciallyvulnerable to sustained jamming periods longer than the velocity memoryservo system of the missile itself. At the cessation of jamming, a rangesearch normally must be conducted in order to place the target againwithin the range gate, for during the jamming interim it is certain thatthe target in moving from its last known position will interrupt thetrackingv coincidence normally required between the target echoes andthe range gate of the missile receiver.

In seeking to establish range lock-on once jamming has stopped, it isaxiomatic that only the range remaining between the radar main bangpulse and the programm'ed maximum flight range of the missile should besearched. in one known system in which the missile is launched into selfflight from an aircraft, the main bang pulses are defined as theplane-emitted radar pulses which illuminate the target following missileand aircraft separation. When received by a rearwardfacing aft-mountedantenna disposed on the missile each main bang pulse establishes theposition of the missile in space as zero search range at the instant themain bang pulse is registered. This approach avoids making a uselessrange search of a large .area beyond the effective flight capability ofthe missile. Thus, at the conclusion of jamming and with the targetreturns no longer coincident with the occurrence of the missile rangegate, the range gate must be repositioned to lock onto the target video.The present invention therefore includes the feature of slewing themissile range gate back and forth over the distance separating themissile and target until coincidence of the target video and the rangegate is achievedJAt this time the missile resumes tracking in the normalmode, i.e., by homing on target echo pulses as the target is illuminatedby the launch plane.

Another object of the invention is to provide a system for obtaining thereacquisition of a target by a missile whose tracking lock on the targetis temporarily dis rupted.

A further object of the invention is the provision of a system whereinthe radar main bang pulse is repetitively used as a zero range referencefrom which a movable range gate is generated.

Other objects will appear hereinafter.

In the drawings, FIG. 1 is a schematic circuit illustrating theoperation of a relock establishing system in accordance with theinvention; and

FIG. 2 shows waveforms useful in understanding the operation of theembodiment illustrated in FIG. 1.

Air-toair missiles of the type in which the incorporation of the FIG. 1embodiment is contemplated employ a front-end mounted guidance systemhaving a dish antenna directed toward r-f energy bounced from a targetilluminated by the launching aircraft during the attack run. Antenna 10,however, is a rearward-looking antenna shielded from thetarget-reflected energy so that it sees only the main bang pulsetransmitted by the launch aircraft. A generator 12 fed by antennaproduces a single pulse output for each main bang carrier wave pulse.The train of positive pulses from generator 12 therefore is slaved tothe main bang pulses. This synchronization with the main bang pulsescontinues as long as the missile maintains an approximate collisioncourse with the target. With each pulse output of generator 12coincident in time with a corresponding main bang pulse, a reference isprovided from which the position of the missile relative to the targetmay be defined at any point in the flight geometry.

The output of generator 12 is applied to a sweep control circuit 14. Thepurpose of circuit 14 is to produce the waveform B of FIG. 2. Thisnegative-going rectangular voltage pulse is initiated coincidently witha pulse from source 12, has a duration many times the pulse repetitioninterval of the source 12 pulses and defines the interval during whichthe radar range gate, in the search mode, moves from zero or minimumrange to maximum range. Any suitable circuit for generating thiswaveform may be used for circuit 14, its design being no part of theinvention. One of the best known and most widely used circuits for thispurpose is the monostable multivibrator, as described for instance inVolume 17, Waveforms, of the Radiation Laboratory Series, pages 166 to17 1. If a monostable multivibrator is used for circuit 14, itsoperation is as follows:

With the monostable multivibrator in its stable state, the next pulse tooccur from source 12 triggers it to its unstable state, generating theleading negativegoing edge of the rectangular wave B. The circuitremains in the unstable state, in which it is insensitive to the triggerpulses from source 12 for an interval determined by the parameters ofthe circuit, as described in the above reference. At the end of thisinterval the circuit automatically reverts to its stable state,generating the trailing positive-going edge of wave B and again becomingsensitive to the trigger pulses from source 12, the next occurring ofwhich initiates a new cycle of operation.

The output of sweep control circuit 14 is applied through resistor 18 tothe base of a slow sweep generator 20 herein shown including an NPNtransistor amplifier 22. The emitter of amplifier 22 is connecteddirectly to a unidirectional negative source having, for example, avalue of 18 volts. A capacitor 24 is connected between the collector andemitter of amplifier 22 for generating an exponential slow sweepvoltage. For protecting the base-emitter junction of amplifier 22 fromexcessive reverse-direction voltage a diode 26 is provided. Thecollector of amplifier 22 is returned through load resistor 28 to aunidirectional positive source also having, for example, a value of l8volts.

Following amplifier 22 is a cathode-follower 30 having a triode 32 whoseplate is connected to a unidirectional positive source of volts. Outputresistor 34 returns the cathode of triode 32 to a unidirectionalnegative source of, for example, volts.

The cathode of triode 32 is connected to a conventional blockingoscillator herein generally referenced 36 and including a comparatortriode amplifier 38 and an iron-core transformer 40 having windings 41,42, and 43. High frequency bypass for amplifier 38 is provided by acapacitor 44. The plate of amplifier 38 is connected by means of winding41 and resistor 46 to the unidirectional positive source of 120 volts.Capacitor 48 and resistor 46 doubly serve as a high frequency filterbetween transformer 40 and the plate supply voltage. Diode 50 clamps thepulse output of oscillator 36 above ground thereby damping negativeovershoot. Resistor 52 serves as a fixed load for transformer 40 toestablish the output impedance for oscillator 36. Through resistor 54the pulse output of oscillator 36 is applied to output terminal 56 andthen, in any manner desired, to a suitable utilization circuit.

In the present embodiment, comparator amplifier 38 of oscillator 36produces a short duration output pulse in response to input triggersignals from cathode follower coincident with other input signalsapplied to its grid through winding 42. To this end, the dotmarked endof winding 42 of transformer 40 is connected to a fast sweep generatorreferenced generally 58 and including an NPN transistor amplifier 60. Acapacitor 62 for generating an exponential fast sweep voltage isconnected between the collector and emitter of amplifier 60. A diode 64couples the base and emitter of amplifier 60 to protect againstexcessive reverse direction voltage.

The collector of amplifier 60 is tied to the unidirectional positivesource through load resistor 66 and variable resistor 68. The timeconstant of the fast sweep generator input to blocking oscillator 36therefore is adjustable. By means of the variable resistance pathoffered by resistor 68, capacitor 62 produces a sawtooth ramp voltagealways considerably shorter in duration than the slow sweep voltagedeveloped by capacitor 24.

Diode 70 provides a low resistance path for dissipating the energyimpressed on capacitor 62 each time an output pulse is generated byoscillator 36. Resistors 71 and 72 and diode 74 form a diode-clippernetwork to establish a maximum input voltage to the base of tran sistoramplifier 60.

Control of the sweep output of amplifier 60 is effected by aconventional bistable multivibrator generally referenced 76, andcomprising PNP transistor amplifiers 78 and 80 arranged incommon-collector configuration. The collectors of amplifiers 78 and 80extend to ground through resistors 81 and 82, respectively. The emittersof amplifiers 78 and 80 are returned to the unidirectional positivesource by means of common resistor 83. Resistors 84 and 85 return thebases of amplifiers 78 and 80, respectively, to the source. Transfer ofthe collector step voltages from one section to the other section ofmultivibrator 76 is due to the parallel resistor-capacitor combinations86-87 and 88-90, as is well known in the art. To allow control of fastsweep generator 60, the collector of amplifier 80 of multivibrator 76 isconnected to the base of amplifier 60 through series-connected resistors91 and 92.

For resetting multivibrator 76, the pulse output of oscillator 36 atwinding 43 is coupled to the base of amplifier 78 of multivibrator 76through resistor 93, the parallel connection of resistor 94 and diode95, and capacitor 96.

For initially triggering multivibrator 76 a line 98 extends from theoutput of generator 12 to the base of amplifier 80. In the quiescentstate, amplifier 80 of multivibrator 76 is conductive whereas amplifier78 is cut off. From conduction in amplifier 80 the voltage acrossresistor 82 provides a positive bias by which amplifier 60 of fast sweepgenerator 58 is made conductive. Capacitor 62 therefore is substantiallycompletely discharged.

Slow sweep generator amplifier 22 is also normally biased conductive,the emitter thereby being rendered considerably negative with respect tothe base thereof. By the same rationale, quiescently capacitor 24 issubstantially completely discharged through amplifier 22.

The operation of the circuit of FIG. 1 to provide a train of range gatepulses referenced successively to the main bang pulses will be moreapparent from the idealized curves of FIG. 2. Referring to curves Athrough F in FIG. 2, the horizontal axis is taken as the time axis andthe vertical axis as the amplitude axis. Ground reference is representedas zero amplitude. FIG. 2A represents a series of pulses found at theoutput of generator 12. Each pulse there shown enjoys time coincidencewith a companion main bang pulse received from the launch aircraft. Theidealized step voltage represented by FIG. 2B is the output of sweepcontrol circuit 14 subsequent to the application of the input voltagefrom generator 12. Although no single and absolute value is reserved inthe present invention for the selection of a maximum search range, amaximum search range of roughly 27,500 feet from the missile isestablished with the maximum delay of the output pulses at terminal 56(waveform F of FIG. 2) relative to their associated trigger pulses fromsource 12 set at 55 microseconds. This means that at commencement of thesweep control voltage of H6. 28, the in-flight position of the missileat this instant is based at zero, i.e., each range search proceeds fromthis instantaneous zero range position.

The base-emitter junction of amplifier 22 is reversebiased by the outputvoltage of sweep control circuit 14. Amplifier 22 is driven fromconduction into cutoff. Capacitor 24 begins to charge exponentially tothe rapidly rising collector voltage of amplifier 22. This change isshown in FIG. 2C. Without inversion, the slow sweep voltage is appliedto the cathode of amplifier 38. The fast sweep voltage from capacitor 62at amplfier 60 commences at the same time. The sharp pulse at the baseof amplifier of multivibrator 76 drives amplifier 80 from conductioninto cutoff. Immediately, as well known in the art, amplifier 78 becomesforward-biased and begins to conduct heavily. The voltage at thecollector of amplifier 80 increases negatively to the point that areverse-bias potential is applied to amplifier 60 of fast sweepgenerator 58. Overcome by this change in bias potential, amplifier 60has a rapid reduction of collector current. The voltage on capacitor 62now rises exponentionally in a positive direction. FIG. 2D shows thefast sweep waveform of generator 58 as it would appear were the sweeppermitted to increase uninhibitedly each cycle to its peak value. Thus,one input to the cathode of amplifier 38 is a positive slow rising rampdeveloped by slow sweep generator. The second input applied to the gridis a positive-going ramp from fast sweep generator 58. It is apparentthat the level to which the fast sweep voltage each time must rise totrigger amplifier 38 into conduction depends on the instantaneous levelof the slow sweep sawtooth from slow sweep generator 20. Stateddifferently, the slow sweep voltage of generator 20 provides foroscillator 36 a reference voltage of progressively increasing amplitude.Since the amplitude of the slow sweep voltage is steadily increasing,this means that the fast sweep voltage must rise to a higher level eachtime to trigger amplifier 38 into conduction and thus produce an outputpulse at terminal 56. For each output pulse of oscillator 36 illustratedin FIG. 2F, the amplitudes of the waveforms of FIGS. 2C and 2D aresubstantially in coincidence.

Further, each output pulse from oscillator 36 is applied to the base ofamplifier 78 of multivibrator 76 to extinguish the fast sweep voltage.Wlth the application of thepositive pulse to its base electrode,amplifier 78 reverts to its non-conducting state. In turn, amplifier 80is induced to conduct once again. The sudden precipitous increase incollector current therein impresses a positive-going voltage onto thebase of amplifier 60 of the fast sweep generator. The voltage oncapacitor 62 is then discharged through amplifier 60 upon its return tothe conductive state. As viewed by the grid of amplifier 38 quiescentconditions are thus reestablished. The cessation of the fast sweepvoltage upon each amplitude coincidence of the fast and slow sweepwaveforms therefore is abundantly clear. This effect is illustrated inFIG. 2E which shows the actual fast and slow sweeps superposed.

Throughout FIG. 2, the vertical dashed lines indicate timecorrespondence. Accordingly, in accordance with the invention, it willbe observed that the crossover points of the fast and slow sweepvoltages occur gradually later in the cycle of the slow sweep ramp. Theresult, of course, is to impose an ever increasing delay between theoutput pulses of oscillator 36 and their re lated main bang pulses.

Upon the termination of the slow sweep control voltage from circuit 14amplifier 22 reverts to the conductive state. Asconduction begins inamplifier 22, capacitor 24 is afforded a discharge path, rapidly doingso through the collector-emitter circuit. Another slow sweep voltage isinitiated by placing amplifier 22 in the cutoff condition.

The variable time constant in the charging path of capacitor 62 providesa means whereby more or fewer output pulses may be obtained during theduration of the slow sweep voltage. Further, capacitor 62 recoverspromptly through amplifier 60 to be ready to produce the next fast sweepwaveform.

In a missile control system as thus outlined hereinabove, it ispresupposed that a reduced range exists between the missile and targetsubsequent to the advent of jamming. Assuming no target detection at thecessation of jamming, one proposed use of the output pulses illustratedin FIG. 2F is to restore the range gate of the missile to timecoincidence with target returns. By thus introducing a progressivelylarger delay between each main bang pulse and the onset of each relatedrange gate pulse, the receptivity of the receiving section of themissile will be stepped outwardly once the main bang pulse occurs, andthen again be retraced to the main bang pulse whereupon another slowsweep cycle is initiated etc. until the shifting range gate brackets thetarget. video returns. Said differently, one advantage embodying theinvention is that the missile range gate may be programmed to stepoutwardly from the zero range position until a target is acquired ormaximum range is reached. When the range gate becomes coincident withthe target video signal, presumably still located between the main bangpulse and the predetermined maximum range, normal tracking will resume.Subsequently received target returns will remain centered within thereceiver range gate.

Although only one embodiment of the invention has herein beenillustrated and described, it will be apparent to those skilled in theart that various changes and modifications can be made herein withoutdeparting from the spirit of the invention or the scope of the appendedclaims.

I claim:

1. In a radar system for searching and tracking objects in space, avoltage comparator circuit comprising: a source of serially generatedtime base pulses having a predetermined pulse repetition frequency, biasmeans coupled to said pulse source and controlled by said pulses forderiving a control voltage extended over a predetermined duration, afirst sawtooth generator biased by said control voltage to produce forsaid predetermined duration a sawtooth voltage encompassing a periodgreater in length than a plurality of periods of said pulse repetitionfrequency, a second sawtooth generator, multivibrator means normallyhaving a first quiescent state and one output level and being switchableto a second state and another output level by each of said pulses, saidsecond sawtooth generator being controlled by said multivibrator means,an input connection between said source and said multivibrator meanswhereby in response to the switching of said multivibrator means fromsaid first to said second state said second sawtooth generator producesa second sawtooth voltage, a comparator having first and second inputelectrodes and an output electrode and characteristics of producing anoutput pulse on said output electrode each time the signalssimultaneously applied to said input electrodes agree in amplitude,cathode follower means for applying said first sawtooth voltage to thefirst input electrode of said comparator, circuit means for applyingsaid second sawtooth voltage to the second input electrode of saidcomparator, means connected between said output electrode and saidmultivibrator means for restoring the initial quiescent state of saidmultivibrator means upon the occurrence of said output pulses, wherebysaid second sawtooth voltage undergoes termination upon each reversal ofsaid multivibrator means from said second state to said first quies centstate, the improvement lying in gradually increasing the delay betweeneach of said output pulses and the related one of said time base pulseswhich spatially precedes it.

1. In a radar system for searching and tracking objects in space, avoltage comparator circuit comprising: a source of serially generatedtime base pulses having a predetermined pulse repetition frequency, biasmeans coupled to said pulse source and controlled by said pulses forderiving a control voltage extended over a predetermined duration, afirst sawtooth generator biased by said control voltage to produce forsaid predetermined duration a sawtooth voltage encompassing a periodgreater in length than a plurality of periods of said pulse repetitionfrequency, a second sawtooth generator, multivibrator means normallyhaving a first quiescent state and one output level and being switchableto a second state and another output level by each of said pulses, saidsecond sawtooth generator being controlled by said multivibrator means,an input connection between said source and said multivibrator meanswhereby in response to the switching of said multivibrator means fromsaid first to said second state said second sawtooth generator producesa second sawtooth voltage, a comparator having first and second inputelectrodes and an output electrode and characteristics of producing anoutput pulse on said output electrode each time the signalssimultaneously applied to said input electrodes agree in amplitude,cathode follower means for applying said first sawtooth voltage to thefirst input electrode of said comparator, circuit means for applyingsaid second sawtooth voltage to the second input electrode of saidcomparator, means connected between said output electrode and saidmultivibrator means for restoring the initial quiescent state of saidmultivibrator means upon the occurrence of said output pulses, wherebysaid second sawtooth voltage undergoes termination upon each reversal ofsaid multivibrator means from said second state to said first quiescentstate, the improvement lying in gradually increasing the delay betweeneach of said output pulses and the related one of said time base pulseswhich spatially precedes it.